Spectropolarimetry of broad Hα lines in radio galaxies: constraints on the geometry of the broad-line and scattering regions

Abstract

We present optical spectropolarimetric observations of three radio-loud broad-line active galactic nuclei: 3C 332, 3C 351 and 4C 73.18, together with new observations of 3C 390.3, which we observed previously. Intrinsically polarized broad Hα emission was detected in 3C 332 and 3C 390.3 at levels of ∼3 per cent and ∼1 per cent, respectively. Significant polarization was not detected in 3C 351, while the polarization measured in 4C 73.18 (∼1 per cent) is consistent with foreground contamination by the galactic interstellar medium. Although 3C 332 and 3C 390.3 both exhibit ‘double-peaked’ broad Hα line profiles, their polarization properties differ significantly. The polarized Hα emission in 3C 332 has a similar profile to that seen in total flux and the continuum and line emission is polarized at 70° to the axis of the radio source. In 3C 390.3, however, the double peaks seen in total flux are suppressed in polarized flux and the Hα line and continuum are polarized at ∼5° to the axis of the radio emission. Comparison of these new observations of 3C 390.3 with data obtained 2 years previously reveals significant variations in both total and polarized flux. The implications of these observations for the relationship between the geometry of the broad-line region and that of the scattering medium are discussed. We argue that models in which the Hα emission arises in a relativistically rotating disc cannot easily explain the observed polarization properties of 3C 332 and, in particular, 3C 390.3. We consider instead a model in which the line emission originates in a bipolar outflow and present calculations showing that the observed polarization properties of 3C 390.3 can be broadly explained by scattering from the inner wall of an obscuring torus, while those of 3C 332 are consistent with scattering by particles situated above the poles of the torus. We outline a scheme in which either toroidal or polar scattering dominates in a given object, depending on the orientation of its radio jet axis to our line of sight.